Resistance welding is a process for joining metals, such as steel, aluminum, titanium, metal-matrix composites, or other sheet metals that involves clamping two or more parts together between two electrodes and passing a series of low voltage, high current pulses through the parts. The welding is performed using a weld machine typically having copper or copper alloy welding electrodes (i.e., a fixed electrode and a movable electrode or two movable electrodes). The electrodes are aligned in opposition to each other. They are positioned around sheet metal layers at a specified spot weld site and clamp and press the layers between the two electrodes. A large electric current is momentarily passed between the opposing electrodes through the electrically resistive metal pressed between them. The sheet metal between the electrodes is briefly melted during current flow and then re-solidified to form an integral weld nugget of suitable diameter at the faying surfaces of the sheet metal layers. In a typical manufacturing operation, many welds are rapidly formed. The goal is to form all of the welds to meet size and strength design requirements, within an acceptable tolerance value, and with minimal internal porosity or discontinuities.
Many resistance welding production procedures consist of establishing the process welding parameters by trial and error. Such parameters typically consist of applied electrode load, heat and time. During production, occasional test coupons are made which are destructively inspected to determine weld nugget size and penetration. This manufacturing procedure has produced different size and strength production welds since the test coupon geometry, surface conditions, etc., may not be representative of the production conditions. Actual production parts are often destructively tested to help circumvent this problem. These destructive tests can provide information about the statistical consistency of a group of welds produced, but they provide no information about the repeatability of any specific weld that isn't destroyed. As a result, manufacturers employing this method of quality control routinely produce bad welds that pass through production undetected. Thus there is a need for an improved method of measuring the repeatability of every weld in order to eliminate the problem of poor welds passing through production undetected.
Welding controllers are used to control the force applied by the movable electrode on the metal surface at the weld site. The controller also is programmed to control the weld current as a function of time. The force applied to the welding site by the electrode and the resistive heat generated by the welding current results in an indentation (caused by the displacement of the electrode) in the softened welded surface of the workpiece at the location of the weld nugget. In setting up the welding machine to produce a series of uniform welds, initial values of suitable electrode force, welding current as a function of time, and the duration that electrode force is maintained after termination of the welding current (hold time) are established for the welding machine or gun and the specific workpieces. While the welding controller can be programmed in an attempt to maintain these values so that the same welds are produced during extended manufacturing operations, weld repeatability can vary due to electrode wear, workpiece gap, weld spacing, part orientation and other geometry variations. Furthermore, weld repeatability can be affected by variations in the operation of the welding machine(s) and external disturbances such as power line fluctuations and cooling water temperature. Thus there is need for an improved method of measuring weld repeatability as the weld is taking place that would allow a multi-variable monitor to detect conditions that would adversely affect the outcome of a weld and, in the case of an adaptive control, use that information to take intelligent corrective actions to prevent poor welds from being produced.
Factory resistance welding facilities typically perform periodic destructive testing of actual production parts or test samples which are supposed to be representative of the actual welding conditions. These quality control procedures are laborious and have an adverse impact on factory productivity as well as making the resistance welding procedure more complicated, and do not ensure complete reliability of the welding process because they provide no information about the welds that are not destroyed. Therefore improved methods to sense welding conditions and provide a reliable measure of weld process repeatability are needed.